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Rural Diary
1 December 2019 Matthew Crampton

Insect control using IPM in forage brassicas

The days of spraying broad spectrum insecticides every time insects are seen in a crop are over. We now have the ability to make informed decisions using an Integrated Pest Management (IPM) approach and achieve better crops.

In previous years if insects were seen in a crop, it was sprayed with a broad spectrum insecticide. This was all that was available to us, but with modern science and more selective insecticides, we now have the ability to alter farming techniques and rotations to discourage pest insects and use selective insecticides as a precision tool to target them. 

Most of the insects in your crops are carnivorous insects that eat other insects, which means most of the insects are beneficial to our crops. These are termed ‘beneficial insects’. What we do on-farm affects the populations of both beneficial and pest insects. An example is caterpillars which are a problem every year in forage brassica crops. Diamondback moth caterpillar populations are affected by the number of eggs that the moth lays. Some weeds flowering in a crop can act as a source of nectar to the diamondback moth, increasing the number of eggs they lay and so the number of caterpillars in your crop. Controlling weeds can help reduce caterpillar populations in your forage brassica crops.

When planning your farm rotations, take into account that pests build up in crop paddocks so don’t plant the same crop in the same paddock year after year and plan a good crop rotation. It is a simple decision and doesn’t usually cost much but can have a big impact on pest populations on your farm. 

Beneficial insects are everywhere and forage brassicas are no exception. Some of the ones I see when walking crops are lacewings, hoverfly larvae and parasitic wasps. All of these are sensitive to broad spectrum insecticides, but by using a product which only targets a pest insect, we can protect these beneficial insects while preventing any further damage to the crop. 

One example of how you find out if your crop contains beneficial insects is to look for parasitized aphids. Parasitized aphids are much larger than healthy aphids because they contain the parasitic wasp which is growing inside them. A large number of parasitized aphids and only a few healthy aphids tells us we won’t need to use an insecticide for aphid control and can then focus on what the caterpillar population is doing, reducing the impact on the environment. 

If you would like to get more information about IPM in your forage brassica crops, contact your local PGG Wrightson Technical Field Representative who has access to the latest information and products to suit an IPM approach.

Controlling problem weeds

Some weed populations build up on-farm over time, and it takes a concerted effort to get on top of them. Examples I have come across in New Zealand are horehound, ragwort, Californian thistle, nasella tussock and yellow bristle grass. Use the following three suggestions to help prevent problem weeds getting established on areas of your farm.

  • Step 1: 
    Control the plants the first time you see them on your farm. The stock yards are a good place to look, also watch feeding areas if you have brought in feed.
  • Step 2: 
    Prevent any new weeds from flowering and seeding. Each plant can produce hundreds to thousands of seeds each year so a small amount of prevention is a good idea.
  • Step 3: 
    Find out the best control options for the target weed. This could be spraying or sometimes manual removal.

If you need help identifying a plant you don’t recognise, contact your local PGG Wrightson Technical Field Representative.


Matthew Crampton

Related Articles

Don't let disease rob your maize crop

01 December 2019

Maize gets a number of diseases and they can all affect the final yield, but one of the most damaging is Northern Leaf Blight (NLB) caused by the fungus Exserohilium turcicum. 
NLB overwinters in infected maize leaves, husks and trash from the previous crop. The NLB fungal spores are produced when environmental conditions are favourable in the spring and early summer; then with the aid of rain splash and wind, NLB infects the new plant growing in the same or neighbouring paddock.

New developing leaves that are exposed to periods exceeding 12 hours of wet or damp conditions and temperatures between 18 to 27 degrees are susceptible to infection from NLB if the fungal spores are present. Then heavy dews and warm humid conditions spread the disease rapidly and move the infection to the lower leaves and cob. The most common and most damaging time for infection in New Zealand is December to January.

Yield loss from NLB is driven by the loss of leaf area. With weather conditions favouring infection from the early tassel stage in the crop, yield can be impacted by as much as 30 percent. With an infection later than this stage, the impact is considerably less and there is an increased risk of lodging.

So what can you do to manage the risk?

  1. Crop rotation
    Growing maize crops back-to-back in the same paddock increases the amount of inoculum passed from one crop to the next, so try to avoid this.
  2. Maize grain
    Crops of maize grown after a maize grain crop are more susceptible to infection than after a silage maize crop because more stova is left after harvest.
  3. Cultivation
    If you fully cultivate after harvesting the maize, burying all of the residue, then there is less plant material to infect. Direct drilling, minimal cultivation and strip tilling leave trash on the surface can lead to an infection risk to the next crop.
  4. Hybrid selection
    All major seed breeders publish the susceptibility of each hybrid to NLB, so when choosing your hybrid, make sure you take this into account.
  5. Planting date
    Late planted crops are at higher risk from developing NLB as there is more inoculum around in the environment from earlier planted and infected crops. The later planted crops are more likely to be at an earlier growth stage when infected.
  6. Fungicides
    There are a few products with activity on NLB but they have to be sprayed either before or at early infection, protecting the plant from infection rather than trying to cure it. The best time to prevent the worst impact of disease is at early tassel stage; usually this has to be done by an aircraft because of the crop height.
  7. Monitor 
    Ensure you monitor the crop through the high risk infection period and growth stage.

For more advice on how to manage and control NLB, contact your local PGG Wrightson Technical Field Representative.


Don't let caterpillars steal your summer feed

01 January 2020

When growing forage crops, there are usually weeds, pests and disease competing, consuming or destroying valuable feed. 

The natural ecosystem keeps pests in check and stops them becoming an economic issue. However, sometimes with favourable conditions, they increase in numbers and start having an economic impact on the crop so need to be managed.

The plantain moth is a member of the carpet moth family and is made up of two species (Scopula rubraria and Epyaxa rosearia). While very little is known about these two species, they are commonly found in New Zealand and Australia and are of the type called “loopers”. The adult moths are quite small with a wingspan of 20 mm, both species look very similar and are light brown in colour with darker brown spots and a dark brown band towards the end of the wings.

With the increase in plantain as a stock feed, either as a stand-alone crop with or without clover, or as a constituent of pasture of another crop, caterpillars are rarely an issue in the establishment year, but numbers can grow to epidemic proportions in the second and subsequent summers. If you experience large numbers of moths through the summer (starting in December), then you can expect a large number of eggs laid which hatch quite quickly into caterpillars with a voracious appetite for plantain leaves. The lifecycle of these caterpillars is short, pupating and emerging as adults to lay more eggs and therefore more caterpillars several times through the summer months. In winter, they largely disappear, probably overwintering in the soil as a pupae.

To monitor for these pests through the summer months, walk through your plantain paddocks in the evening. If there are large numbers of small brown moths flying up in front of you, then look closer in the leaf litter and see if you can find small curled up caterpillars and evidence of their feeding on the plantain leaf. In years where there have been large numbers of moths, some farmers have observed 90 percent leaf area of plantain plants consumed by these caterpillars.

If you have identified moths in your crop, contact your local PGG Wrightson Technical Field Representative for a control plan.

Controlling diamondback moth

01 January 2020

Mild winters can result in higher insect pressure in spring and if these conditions turn into a hot and dry summer, then Diamondback Moth (DBM) thrives. Unfortunately, this can mean damage to the leaves of valuable winter feed crops.

DBM is a pest which feeds on brassica plants including forage crops. In a hot dry year, DBM can build to plague proportions and destroy leaf area of feed crops. Using what we know about its lifecycle and beneficial insects, we can make better decisions to reduce the impact of this pest.

The lifecycle of DBM consists of four stages:

  • Adult
  • Egg
  • Larvae (caterpillar)
  • Pupae

The damaging stage is stage three. The caterpillar has the job of eating as much leaf material as it can to allow it to pupate into the adult moth. Just like a monarch butterfly, the caterpillar builds a pupae around itself and transforms into the adult inside it. This eating results in all of the visible leaf damage that we see. 

Knowing which stage causes all the damage allows us to target control options and minimise these effects. The last few years have seen the introduction of two new insecticides which are safer for the applicator and on beneficial insects than previous chemistry. Beneficial insects help us with controlling crop pests by feeding on the pests themselves, so we need to protect them wherever possible. An example of this is the lacewing. The larval stage of the lacewing walks around the leaf feeding on aphids on the crop.

Monitor your crop for the first signs of DBM. If a selective pesticide needs to be applied, the timing should be based on identification of the DBM and lifecycle stage. Identification of this first incursion is hard because the eggs are small and the adult moth doesn’t sit still long enough to easily identify the distinctive diamond pattern on its back. An easier way to identify if the pest is present is visible damage to the crops foliage. DBM damage can be easily identified by the distinctive windows formed in the leaves and/or holes which are away from the edge of the leaf. This is when to time your application for maximum benefit. As the caterpillars ingest the plant material, they also ingest the insecticide. 

If dry conditions continue through late summer, more than one application may be required. Each adult can lay more than 100 eggs, so keep an eye out for another flight going through the crop then monitor for another batch of eggs hatching. 

To get the best control of DBM, contact your local PGG Wrightson Technical Field Representative. 


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